Spectrometers are used in many applications for measuring properties of light across a range of wavelengths. For example, a spectrometer can be used for compositional analysis, by obtaining absorption or emission spectra for an object of interest. The presence and location of peaks within the spectra can indicate the presence of particular elements or compounds. Spectrometers are commonly used for analysis at optical wavelengths, but can also be used at other wavelengths such as microwave and radio wavelengths.
Spectrometers are typically relatively complex and expensive devices that require the alignment of a number of moving parts to be controlled with high precision. For example, a typical spectrometer may focus light onto a diffraction grating to split an incident beam into separate wavelengths, and the diffraction grating may be rotated to a specific angle to direct light of a particular wavelength towards a detector. In recent years chip-based spectrometers have been developed which can be highly miniaturised, have no moving parts, and can be manufactured using well-established lithography techniques. An example of such a spectrometer-on-a-chip is shown in FIG. 1.
The chip spectrometer 100 comprises a substrate 110, onto which are patterned a waveguide 120 and a plurality of disk resonators coupled to the waveguide. The waveguide 120 guides the input light to the disk resonators. Light is input to one end of the waveguide, and each resonator 130 is arranged to support a resonant mode at a particular wavelength such that only light of that wavelength is coupled into the resonator 130. On top of each disk resonator 130 is an electrode 140 for detecting current that is proportional to the amount of light present in that resonator. The current detected in each resonator therefore indicates the amount of light at that wavelength that was present in the input beam of light. Each electrode 140 is further connected to a signal bond pad 150 for connecting the spectrometer 100 to an external device for measuring the current. The light input to the waveguide 120 has to be closely aligned to the centre of the waveguide 220, in order to achieve optimum coupling into the waveguide 220.